Field of the Invention
The present invention relates to an image pickup apparatus capable of selectively picking up a color image and a black-and-white image (monochrome image). In particular, the present invention is preferred for an image pickup apparatus, such as a surveillance camera, which is demanded to exhibit a high resolution at a time of picking up a color image and exhibit a high sensitivity at a time of picking up a black-and-white image.
Description of the Related Art
In recent years, a surveillance camera has been demanded to increase accuracy of an image to be obtained. For example, pickup of a color image of a target object allows not only shape recognition of the object but also acquisition of more information on the target object, such as a kind thereof or a feature thereof, from abundant color information, which facilitates identification of the target object. Hitherto, many image pickup apparatus often employ a single-plate type to obtain a color image by using one image pickup element. An image pickup apparatus of the single-plate type is configured to obtain the color image by providing a color mosaic filter in front of the image pickup element (between the image pickup element and a lens for forming an object image) (Japanese Patent Application Laid-Open No. 2013-223152).
FIG. 9 is an exploded perspective view of an image pickup unit 10 including a solid-state image pickup element used for a related-art color camera of the single-plate type. The image pickup unit 10 includes a package 11, a solid-state image pickup element 12, a mosaic filter 33 for color separation, and front glass (cover glass) 14. The mosaic filter 33 is configured such that a green (G) filter is located for two pixels among every four pixels, a red (R) filter is located for one pixel of the remaining pixels, and a blue (B) filter is located for the last remaining pixel.
In the resolution of the color camera of the single-plate type, a contribution ratio of the pixel for green (G) is large and contribution ratios of the pixels for red (R) and blue (B) are low. Thus, the color camera of the single-plate type obtains the color image at the cost of the resolution of the image pickup element intrinsic to the image pickup unit 10.
In addition, there is known a color camera of a three-plate type obtained by combining a color separation optical system of a prism type and three image pickup elements in order to obtain the color image (Japanese Patent Application Laid-Open No. 2003-309858). Recently, the color camera of the three-plate type using a solid-state image pickup element easy to handle is often used, and the color camera of this type causes less degradation in the resolution than the color camera of the single-plate type using the one image pickup element. Further, the color camera of the three-plate type has a feature of exhibiting high energy utilization efficiency of light because the light is split into a plurality of wavelength regions to be received by the respective image pickup elements.
The image pickup apparatus including one image pickup element provided with the color mosaic filter in order to obtain the color image has a simplified overall configuration, but has a lower resolving power than the resolution exhibited by the single image pickup element. When a photographing area (photographing field angle) is set wider, the surveillance camera is enabled to monitor a wider range with a smaller dead angle, which increases safety. However, the lowered resolution lowers a shape recognition rate even when the color information on the target object can be recognized. Therefore, the photographing area is hard to widen in order to obtain the photographing area suitable for the resolution.
In order to monitor a suspicious ship or the like at a longer distance, the surveillance camera used for harbor surveillance or the like is required to use a photographing lens having a longer focus distance. However, when the resolving power is lowered, it becomes difficult to recognize an object at a distance with high accuracy.
The color camera of the single-plate type is not originally high in light utilization efficiency. One of the factors therefor is that the mosaic color filter conducts spatial color separation as described above. Another of the factors is that light energy utilization efficiency is lowered when the mosaic color filter itself absorbs not only unnecessary light but also light having a necessary wavelength band.
The surveillance camera is desired to be capable of photographing with small noise and a high resolution during photographing under a low illuminance, for example, during nighttime. However, the light utilization efficiency is not high as described above, and hence incident light energy with respect to the image pickup element is small under a low illuminance, which leads to a tendency that noise becomes larger with the lowered resolving power.
In the color camera of the three-plate type, the spatial color separation is not conducted, and hence the resolving power of the image pickup element is maintained, which facilitates prevention of degradation in the resolution. However, the color camera of the three-plate type tends to become lower in the resolving power. The color camera of the three-plate type uses light energy more efficiently as the color camera than the color camera of the single-plate type. However, the light is split based on the wavelength to enter the image pickup element, and hence energy of light entering one image pickup element is not 100%. For example, the light entering the color separation optical system has an energy distribution only within a visible range.
In the color separation optical system used for the color camera of the three-plate type, the light is split into three wavelength bands of a green wavelength band, a red wavelength band, and a blue wavelength band. That is, the energy of the incident light is guided to the image pickup element by being divided into approximately ⅓ for each color of the light. However, under a low illuminance that does not exhibit a sufficient light amount, the energy of the light entering the individual image pickup element is small with large noise, which lowers the resolving power of the image pickup element. In particular, there is a problem in that a signal-to-noise ratio does not improve even when signals from the three image pickup elements are added up.
The surveillance camera of the single-plate type or the like may obtain a higher resolving power by obtaining a black-and-white image than by picking up a color image when the object whose image is to be picked up has a low illuminance.
On the other hand, when the object whose image is to be picked up has a high illuminance, the pickup of the color image through use of a plurality of image pickup elements allows the color information on the object to be obtained as well while alleviating lowering of the resolving power, and allows improvement of the recognition rate. However, it requires a complicated configuration, and is considerably difficult, to selectively pick up the color image and the black-and-white image between when the object whose image is to be picked up by one surveillance camera has a low illuminance and when the object has a high illuminance.